The field of the invention relates generally to power converter systems for supplying power to a permanent magnet motor.
Commonly used in power generation systems, converters are devices that either convert alternating current (AC) power into direct current (DC) power (rectifier), or DC power to AC power (inverter). Whether a converter is being used as a rectifier or an inverter is dependent on the direction power is flowing through the converter. A typical power generation system may include a generator for producing AC power, an AC-DC converter (or rectifier), a DC link, a DC-AC converter (or inverter), and a load. The AC power generated by the generator is provided to the rectifier, which provides a rectified signal to the DC link. The inverter uses the rectified signal provided to the DC link to generate the desired AC power to the load. The quality of the AC power generated by the inverter is related to the quality of the DC power provided to the DC link. That is, by reducing fluctuations in current on the DC link, commonly referred to as ripple, the overall performance of the system is improved.
A typical method of reducing ripple on the DC link is to provide a capacitor or other filtering device between the rectifier and the inverter. The size of the capacitor or filter selected to reduce ripple is based on the amount of ripple that must be reduced. As ripple currents become large, the size of the capacitor becomes increasingly large and costly. Therefore, it would be desirable to design a system that reduces ripple current without the prohibitive cost of a large capacitor. Furthermore, the capacitor used to reduce ripple current between the rectifier and the inverter is often subject to high instantaneous currents at the initialization of the system, which adversely affects the performance of the capacitor. Therefore, it would also be desirable to design a system that would reduce exposure of the capacitor to high instantaneous currents.
The rectifiers discussed above can be generally classified as non-linear loads, since they may draw non-linear currents from the AC power source. A generator providing the AC power to the rectifier may also provide AC power to a number of other non-linear loads. Each of these non-linear loads contributes to generate harmonic currents on the AC input lines. A typical method of reducing the harmonic currents on the AC input lines is to include capacitive and inductive elements at the input of each non-linear load. Again, the size of the inductive and capacitive elements is related to the magnitude of the harmonics to be reduced. It would therefore be desirable to provide an economical solution to reducing the harmonic currents created on the AC input lines by non-linear loads.